Necessity of Vacuum Relief Valve - Not Pressure Relief Valve

My new water heater states: "Install vacuum relief in cold water inlet line as required by local codes". My inspector says I need one. I read on one of the old posts that you only need one if your water heater is on an upper floor or in the attic. Mine is in the attic. What is the purpose of a vacuum relief valve? What could possibly happen if I didn't install one - other than the inspector killing me?

It prevents siphoning of the water heater.
If the water main is shut off to the home, and then a faucet is opened, it will siphon the water heater. When that happens, the top element burns out in seconds. With a non-functioning top element, you now have no hot water.

I have not heard that one in a long time....
Inside most heaters today there is a port hole on the
dip tube inlet at the top of the tank that prevents the
heater from siphoning....... a heater cannot siphon down

quote; Inside most heaters today there is a port hole on the
dip tube inlet at the top of the tank that prevents the
heater from siphoning....... a heater cannot siphon down

the valve they want you to install is not necessary

It is NOT to prevent siphoning. If the water supply to the building is turned off and the water is drained out, the tank will be subjected to "severe" negative pressure, i.e., a vacuum, and the amount of the negative pressure will be in direct corellation to how high the heater is above the drain point. The heater will withstand 300 psi of pressure but relatively LITTLE negative pressure so it will "collapse". If you have you ever seen a tanker truck that the driver opened the drain hose before he opened the lids, you have seen a collapsed tank and that was just the suction of a few feet between the truck and the ground. In Texas there was a city water storage tank that "imploded" and it was VERY impressive to see it look like an accordian. If you heater is in the attic, you DO need the vacuum relief valve, just as a safety measure.

Ja, hj has the right answer. A vessel designed only for pressure should not be exposed to a vacuum. There have been documented cases of various tanks collapsing from the improper implementation of a booster pump as well. Water softeners have also collapsed from an improper drain line installation.

THey're required where I live, regardless of where it is installed, as is a tempering valve. Really pisses off uninformed people when it comes time to replace their WH (assuming they get a permit), as it can add some to the installed price.

That is the problem with beaurocrats, a special interest tells them how to solve a certain "occassional problem" and they decide to make it a universal solution. We have the same thing here with pressure reducing valves. Most areas do not have the pressure that would require them, but since they do not want to address just the areas where a PRV is needed, they just passed an ordinance that EVERY house must have one.

I dont have a clue as to what you guys are referring to
so please inform me as to what this is all about ...are you speaking about
residential or commercial systems actully imploding??

I have yet to see a water heater on the higher floors of any building
"implode" because they have drained down all the water....
Inside all heaters there is a dip tube with a anti siphon port hole
to keep this from happenning...... and I doubt it could

I would like to have an imploded heater for my store front window
if one actually exists..... that would be an extremely rare find.....

back a long time ago , this was something that was required on
drain down solar panels. I had to tangle with some "solar enjineers"
on this subject becasue they were so un-reliable that they would
leak and had to be changed out all the time due to any pressure fluctuations
in the system.......

this vaccuum relief valve will only cause troubles and probably leak all
over someones attic yearsdown the road when the system is turned off when
the poor plumber attempts to change out a ballcock or hose bib.......

Not a water heater but the same thing happens on a smaller scale with a water heater, since it also does not have internal baffles, (which pressure vessels such as a sumbarine do have to resist collapse from exterior pressure), to strengthen it.http://www.ebaumsworld.com/video/watch/344750/?autoplay=true

MPM, the "hole" is to prevent siphonage, NOT suction. Siphonage will only occur if a faucet is opened to let air into the system. Vacuum depends on the entire system being closed/sealed. In fact, if a faucet were opened so the hole stopped siphonage, it would ALSO break the suction and eliminate the need for a vacuum relief valve.

Mark,
Often when I'm draining down the house to do a pipe repair or to replace shutoffs, I loosen the flex connectors on the water heater to prevent the tank from siphoning. If it's electric, I just turn off the breakers at the panel so I don't let the water drop too low and burn out the top element.

Siphoning happens. We just take the right steps, knowing that.

In the city of Bellevue, if the electric heater is on the first floor, the vacuum breaker is not required, only if it's on a second or higher story.

Mark,
Often when I'm draining down the house to do a pipe repair or to replace shutoffs, I loosen the flex connectors on the water heater to prevent the tank from siphoning. If it's electric, I just turn off the breakers at the panel so I don't let the water drop too low and burn out the top element.

Siphoning happens. We just take the right steps, knowing that.

In the city of Bellevue, if the electric heater is on the first floor, the vacuum breaker is not required, only if it's on a second or higher story.

Water heaters will siphon!

Click to expand...

Terry ...you have a firm grip on the obvious

I completely agree with you terry..... yes they will siphon
till they hit the siphon hole on the dip tube in the heater..
and sometimes its not a bad idea to open the t+p valve
or something like you stated

YES a water heater will siphon,

but unless you put the heater to a test in an experiment
to see how much negative pressure you need to use to make it
implode..... the odds are it will never , ever implode. in a real world
situation.....,,,

I am gonna go get my power ball tickets now...
its up to 169 million and my odds are better to win
the power ball than see a water heater implode in my lifetime...

on another note..... how have you been doing lately???
feeling ok and all???.

I'm not worried about one imploding.
Hey, I like this big text.
Feeling good. I spent a few days doing a repipe in a crawl. That gave my stomach muscles a workout. Just waiting for snow to fall so I can start using my skis.

jadnashua, thank you for your post ref. New England code req's.
Do you have any diagrams or pics of approved N.E. installations with both fittings in place?
I ask because diagrams and pics on the web show all manner of confusing arrangements where tempering valves are installed in residential situations.
I'm going to post a new/separate message about this tempering valve business.

Suction is a function of gravity AND air pressure. Water falling down, in a closed space, creates a loss in pressure. If there is no way for air to get into the chamber, a negative pressure, aka a vacuum, occurs, but siphonage does NOT occur. If air can enter, then depending on the dynamics of the chamber, the liquid either drains, or if it is part of an inverted "U" shape then it is siphoned out until it reaches a level where air can enter and break the flow. This point is either the "anti siphon" hole or the bottom of the siphon tube. In all cases, air entry is required to enable the system to reach an equilibrium. The vacuum relief valve is installed at the highest point in the system. Tempering valves can be installed anywhere they are needed. Hot in on one side, cold in on the other, and the tempered water out of the center to the system. They are installed anywhere you have access to all three lines.